The prime minister is under doctor's orders to rest for 24 hours after hospital treatment for an irregular heartbeat.

Tony Blair cancelled a planned speech in the House of Commons, although he is going ahead with meetings at Downing Street.

Foreign Secretary Jack Straw is to deliver the statement on the EU summit in the Commons in his place.

The 50-year-old has never suffered heart problems before but was taken to London's Hammersmith Hospital on Sunday, after complaining of chest pains at his Chequers residence and first going to Stoke Mandeville Hospital.

Doctors diagnosed supra ventricular tachycardia - a condition that causes heartbeat irregularities and shortness of breath - and ordered the father-of-four to rest for a day.

He was kept in hospital for more than five hours and treated with a procedure called cardio version, which uses a small electric shock to make the heartbeat return to normal.

The prime minister has spent the past few months battered by the greatest pressures of his entire political life

"Any health incident involving a prime minister in office has got to be an issue for perfectly good and serious reasons, but that obviously doesn't translate into there being any indication that it is anything more serious."

A Downing Street spokesman said: "The hospital says this is a relatively common condition and is easily treated.

"He has suffered no damage and he is fine. There is no reason why this should reoccur. They have advised him to rest for 24 hours."

Mr Blair has been under a great deal of stress in recent months, particularly after the strains of the war in Iraq.

'Frightening'

But experts said the condition was more likely to be something that simply affected some people rather than others, instead of being linked to stress.

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"It can be very frightening for people when it happens, particularly when they don't know what it is."

He said it could happen to healthy people, and once treated was "compatible with a completely normal quality and quantity of life".

Dr Dymond said Mr Blair should have a couple of days rest, but added: "It's by no means set in stone that he will have another attack and there's no real reason for him to stop playing tennis or stop being prime minister based on this."

The US administration was "very glad to hear that he [the Prime Minister] is doing well", National Security Advisor Condolezza Rice told reporters travelling with President George W Bush in Thailand.